The present invention relates to a filter circuit such as a first-order low-pass filter, a first-order high-pass filter, or a first-order all-pass filter, and particularly to a filter circuit with a wide dynamic range capable of low-voltage operation.
As a conventional example of a filter circuit, for example a first-order low-pass filter with a wide dynamic range capable of low-voltage operation, a filter circuit disclosed in Japanese Patent Laid-Open No. Hei 9-69752 is known, for example. A circuit configuration of the first-order low-pass filter according to the conventional example is shown in FIG. 13.
In FIG. 13, a base electrode of a transistor Q1 is connected to a circuit input terminal 101 of one of differential inputs. A collector electrode of the transistor Q1 is connected to a power supply line 103 of a supply voltage VCC. An emitter electrode of the transistor Q1 is connected with an emitter electrode of a transistor Q2. The transistor Q2 is of a diode-connected configuration, in which a base electrode and a collector electrode of the transistor Q2 are connected to each other. A current source 111 is connected between a GND line 104 at a ground level and a common emitter connection point of the transistors Q1 and Q2.
The base electrode and the collector electrode of the transistor Q2 are connected with a base electrode and a collector electrode of a transistor Q3. Thus, the transistor Q3 is also of the diode-connected configuration, and is connected in parallel with the diode-connected transistor Q2 with a polarity opposite from the transistor Q2. A current source 112 is connected between the power supply line 103 and a common connection point of the bases and the collectors of the transistors Q2 and Q3. An emitter electrode of the transistor Q3 is connected with an emitter electrode of a transistor Q4. A current source 113 is connected between the GND line 104 and a common emitter connection point of the transistors Q3 and Q4.
By thereafter repeating the same connecting relation, a total of n transistors Q1 to Qn are connected to one another. Then, a current source 114 is connected between the GND line 104 and a common emitter connection point of an (nxe2x88x921)th diode-connected transistor Qnxe2x88x921 and an nth diode-connected transistor Qn in a final stage. A current source 115 is connected between the power supply line 103 and a common connection point of a base and a collector of the transistor Qn. The common connection point of the base and the collector of the transistor Qn is also connected to a circuit output terminal 105 of one of differential outputs, and connected to one terminal of a capacitor 107.
A circuit formed by n transistors (transistors Q2n to Qn+1) and current sources in exactly the same connecting relation as the above circuit is connected between a circuit input terminal 102 of the other differential input, a circuit output terminal 106 of the other differential output, and the other terminal of the capacitor 107. It is to be noted that the first-order low-pass filter according to the present example is an example of a circuit when n is an even number; when n is an odd number, the connecting relation of the nth transistor Qn (Qn+1), the circuit output terminal 105 (106), and the one terminal (other terminal) of the capacitor 107 is as shown in FIG. 14.
A circuit equivalent to the thus formed first-order low-pass filter according to the conventional example is shown in FIG. 15. As is clear from the equivalent circuit, the first-order low-pass filter has a circuit configuration in which n emitter resistances re of the transistors are connected in series with each other between the circuit input terminal 101 and the circuit output terminal 105 and between the circuit input terminal 102 and the circuit output terminal 106, and the capacitor 107 is connected between the circuit output terminals 105 and 106.
Letting vi be an input signal, vo be an output signal, I be a current flowing in each of the transistors, C be capacitance of the capacitor 107, and s be a complex frequency, a transfer function H (=vo/vi) of the first-order low-pass filter is:
[Equation 1]                    H        =                              1                          2              ⁢                              re                ·                n                ·                C                                                          s            +                          1                              2                ⁢                                  re                  ·                  n                  ·                  C                                                                                        (        1        )            
The emitter resistance re is expressed as re=Vt/I, where Vt=kT/q, k being the Boltzmann constant, T being the absolute temperature, and q being the amount of electron charge. The cut-off frequency fc is:
fc=xc2xcxcfx80xc2x7rexc2x7nxc2x7C
As is clear from FIG. 13 and FIG. 14, because of the circuit configuration in which only two current sources and one transistor circuit are arranged between the power supply line 103 and the GND line 104, the first-order low-pass filter according to the conventional example has advantages of being able to operate at a low supply voltage and extend the input dynamic range by a factor of n by increasing the number n of transistors.
However, in the first-order low-pass filter formed as described above according to the conventional example, the extension of the input dynamic range requires an increase of the number n of transistors, and hence when the cut-off frequency fc and the capacitance C of the capacitor 107 are fixed, the increase of the number n of transistors results in an exponential increase in current consumption in accordance with the number n.
The present invention has been made in view of the above problem, and it is accordingly an object of the present invention to provide a filter circuit of low-voltage operation that can extend the input dynamic range while reducing current consumption.
In order to achieve the above object, according to the present invention, there is provided a filter circuit comprising: a first differential circuit formed by a combination of one transistor and four diodes connected in parallel with each other and each having one electrode connected to a first electrode of the transistor, a first current corresponding to an input signal flowing through the four diodes; and a second differential circuit formed by a combination of one diode and four transistors connected in parallel with each other and each having a first electrode connected to one electrode of the diode, a second current corresponding to the input signal flowing through the one diode. Further, a current source is connected to a common connection node of the four diodes and the one diode. A capacitor through which a current determined by a current of the current source and the first and second currents flows is connected to predetermined nodes, whereby a low-pass filter, a high-pass filter, or an all-pass filter is formed.
Hereinafter, bipolar transistors will be taken as an example of the transistors forming the first and second differential circuits. In this case, the first electrode of the transistor refers to an emitter electrode for injecting a carrier (electron or hole); a second electrode refers to a collector electrode reached by the carrier; and a control electrode refers to a base electrode supplied with a current for controlling movement of the carrier injected from the emitter electrode. The one electrode of a diode refers to a cathode electrode, and when the diode is formed by a transistor, the electrode refers to an emitter electrode.
By providing the thus formed filter circuit with the first differential circuit in which a ratio of the number of transistors to that of diodes is 1:4 and the second differential circuit in which the ratio of the number of transistors to that of diodes is 4:1, and by connecting the current source to the connection node of the diodes, the differential circuits have two operating points. By adding together the first and second differential circuits having two operating points, it is possible to extend the dynamic range. In addition, the cut-off frequency is made variable by changing the current of the current source.